MITSUBISHI MONTERO GAS PROBLEM

2002 Mitsubishi Montero • 129,000 miles

Had heads done, all new gaskets put in, new water pump and timing belt and cerpitine belt put on. Just had all work done in2012oct. While sitting in drive and driving truck pulll real vad. Also if I get over 45miles an hr it starts to slow siwn. What could be the problem?